This invention relates to injection molding machines in general, but particularly to the reciprocating screw or plunger type machine suitable for the injection molding of plastic and elastomeric materials. Conventional molding apparatus of the reciprocating-rotating screw type usually includes a plasticizing cylinder or chamber having a bore, wherein the plasticizing screw rotates in such a manner so as to allow the solid molding material to enter the cylinder and be plasticized as it advances in the direction of screw feed. Attached on one end of the plasticizing cylinder is a nozzle in communication with a mold sprue. As the plasticized material is deposited at the metering or front end of the screw, it developes a back pressure that forces the screw to retract in the cylinder bore and when the plasticized material reaches a predetermined volume, or shot size, the retracting screw contacts a limit switch and stops its rotation. At this stage, the shot is ready for injection into the mold, generally upon receipt of a signal from the clamp, whereupon the screw is driven forward hydraulically to inject the shot. Upon receipt of another signal, the plasticizing screw again starts to rotate and gradually retract as a fresh shot is built up in the plasticizing cylinder. Thus, the screw reciprocates once per machine cycle to plasticize and inject a shot of material.
Often, shut-off valves or devices are employed to interrupt the flow of molten material from the nozzle into the mold sprue. These offer the advantages of minimizing or entirely curtailing drool through cut off of material flow at the nozzle and provide the capability to plasticize during periods in which the clamp is open. Generally, plasticizing takes place during part curing to prevent plasticized material from escaping.
Over the years many types of shut-off devices and means to actuate them have been developed and utilized with varying degrees of success. Included among these are, for example, spring operated needle valves such as shown in FIG. 4 of U.S. Patent 3,002,229, which are generally not completely reliable in actual operation; mechanically operated pin valves such as shown in U.S. Patent 3,172,161 which are generally difficult to properly align and have multiple pivot points subject to wear; hydraulically operated rotary valves such as shown in FIG. 3 of U.S. Patent 3,169,275 which are often subject to scoring and difficult to locate near the nozzle because of high nozzle temperatures which make hydraulic operation hazardous; .[.and.]. hydraulically operated piston valves such as shown in FIG. 3 of U.S. Patent 3,026,567 which again are difficult to locate near the nozzle due to the high nozzle temperatures involved.[...]..Iadd.; and a pneumatic piston operated valve with a floating seal on a reciprocable valve gate member is described in U.S. Patent 2,936,996. .Iaddend.